在UNIX上如何使用C日期和时间函数？ [英] How do I use the C date and time functions on UNIX?
问题描述
p>
术语
日期/时间可以有两种格式： / p>
- 日历时间（aka simpletime） - 时间作为绝对值，通常是由于某些基准时间（通常称为协调世界时）
- 本地时间（又称分解时间） - 由年，月，日等组成的日历时间，考虑到本地时区，包括夏令时（如适用）。
数据类型
日期/时间函数和类型在当时声明。 h头文件。
时间可以存储为一个整数或一个结构的实例：
-
使用 time_t 算术类型 - 将日历时间存储为自UNIX时代以来经过的秒数1970年01月01日00:00:00
-
使用结构 timeval - 将日历时间存储为自UNIX时代以来的秒数和纳秒数1970年01月01日00:00:00
-
使用结构 tm 存储本地时间，它包含以下属性：
tm_hour
tm_min
tm_isdst
上面的tm_isdst属性用于指示夏令时（DST）。如果值为正值，则为DST，如果值为0，则不是DST。
打印当前协调世界时间的程序
#include< stdio.h>
#include< time.h>
int main（int argc，char * argv []）
{
time_t now;
now = time（NULL）;
printf（从1970年1月1日00:00:00开始的％ld秒，（长））
return 0;
}
在上面的程序中，时间读取UNIX系统时间，从1970年1月1日00:00:00减去（UNIX时代），并在几秒钟内返回结果。
打印当前本地时间的程序
#include< stdio.h>
#include< time.h>
int main（int argc，char * argv []）
{
time_t now;
struct tm * lcltime;
now = time（NULL）;
lcltime = localtime（& now）;
printf（时间是％d：％d\\\
，lcltime-> tm_hour，lcltime-> tm_min）;
return 0;
}
在上面的程序中， localtime 将从UNIX时期开始的经过时间（秒）转换为分解时间。本地时间读取UNIX环境TZ（通过调用tzset函数）返回相对于时区的时间并设置tm_isdst属性。
UNIX中的TZ变量（使用bash）将如下所示：
export TZ = GMT
或
export TZ = US /东方
打印当前格式格林威治标准时间的程序
#include< stdio.h>
#include< time.h>
int main（int argc，char * argv []）
{
time_t now;
struct tm * gmt;
char formatted_gmt [50];
now = time（NULL）;
gmt = gmtime（& now）;
strftime（formatted_gmt，sizeof（formatted_gmt），％I：％M％p，gmt）;
printf（时间是％s\\\
，formatted_gmt）;
return 0;
}
在上面的程序中， strftime 提供专门的日期格式。
要考虑的其他问题
Jon Skeet spoke of the complexity of programming dates and times at the 2009 DevDays in London.
Can you give me an introduction to the ANSI C date/time functions on UNIX and indicate some of the deeper issues I should also consider when using dates and times?
Terminology
A date/time can be in two formats:
- calendar time (a.k.a. simpletime) – time as an absolute value typically since some base time, often referred to as the Coordinated Universal Time
- localtime (a.k.a. broken-down time) – a calendar time made up of components of year, month, day etc. which takes into account the local time zone including Daylight Saving Time if applicable.
Data types
The date/time functions and types are declared in the time.h header file.
Time can be stored as a whole number or as an instance of a structure:
as a number using the time_t arithmetic type – to store calendar time as the number of seconds elapsed since the UNIX epoch January 1, 1970 00:00:00
using the structure timeval – to store calendar time as the number of seconds and nanoseconds elapsed since the UNIX epoch January 1, 1970 00:00:00
using the structure tm to store localtime, it contains attributes such as the following:
tm_hour tm_min tm_isdst
The tm_isdst attribute above is used to indicate Daylight Saving Time (DST). If the value is positive it is DST, if the value is 0 it is not DST.
Program to print the current Coordinated Universal Time
#include <stdio.h>
#include <time.h>
int main ( int argc, char *argv[] )
{
time_t now;
now = time ( NULL );
printf ( "It’s %ld seconds since January 1, 1970 00:00:00", (long) now );
return 0;
}
In the program above the function time reads the UNIX system time, subtracts that from January 1, 1970 00:00:00 (the UNIX epoch) and returns its result in seconds.
Program to print the current local time
#include <stdio.h>
#include <time.h>
int main ( int argc, char *argv[] )
{
time_t now;
struct tm *lcltime;
now = time ( NULL );
lcltime = localtime ( &now );
printf ( "The time is %d:%d\n", lcltime->tm_hour, lcltime->tm_min );
return 0;
}
In the program above the function localtime converts the elapsed time in seconds from the UNIX epoch into the broken-down time. localtime reads the UNIX environment TZ (through a call to the tzset function) to return the time relative to the timezone and to set the tm_isdst attribute.
A typical setting of the TZ variable in UNIX (using bash) would be as follows:
export TZ=GMT
or
export TZ=US/Eastern
Program to print the current formatted Greenwich Mean Time
#include <stdio.h>
#include <time.h>
int main ( int argc, char *argv[] )
{
time_t now;
struct tm *gmt;
char formatted_gmt [50];
now = time ( NULL );
gmt = gmtime ( &now );
strftime ( formatted_gmt, sizeof(formatted_gmt), "%I:%M %p", gmt );
printf ( "The time is %s\n", formatted_gmt );
return 0;
}
In the program above the function strftime provides specialised formatting of dates.
Other issues to consider
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